This invention relates to metal forming and joining, and particularly to superplastic forming and diffusion bonding of nickel base alloys.
Nickel base alloys such as Inconel 718 alloy have been used extensively in high-temperature structural applications because of an advantageous combination of properties. However, there is a continuing need to develop improved and less costly methods of fabricating these materials.
Some alloys such as titanium based alloys can be fabricated into parts by superplastic forming. However, conventional Inconel 718 has grains which are too large to permit the use of superplastic forming (See Journal of Spacecraft and Rockets, vol 21, No 1, Jan.-Feb., 1984, "Superplastic Forming and Diffusion Bonding of Inconel 718", by W. T. Chandler, et al). Consequently, a special fine grain Inconel 718 alloy with a grain size less than 9 microns has been developed. This fine grain alloy can be superplastically formed to elongations up to about 200%. An even finer grained version (about 6 microns) has superplastic capabilities of up to 500 to 600% elongation.
Unfortunately, the fine grain material cannot be diffusion bonded and then superplastically formed because the high temperature required for prior art diffusion bonding causes the fine grains to grow rapidly and thus destroys the superplastic properties of the material. This limits the usefulness of both prior art processes, because superplastic forming cannot be done after diffusion bonding or concurrently with diffusion bonding (such as described for titanium alloys in U.S. Pat. No. 3,920,175).